1. Field of the Invention
This invention relates to the identification of fibers of fiber optic cables. Typically a fiber optic cable comprises many individual fibers. Typically a fiber optic cable extends for several kilometers between regeneration stations which boost the signals travelling down the cable.
2. Summary of Prior Art
If a fiber optic cable is broken, for example by excavation, the cable operating company will want to re-connect the cable as soon as possible. This will involve re-connecting the many individual fibers in the length of broken cable on a first side of the break with the many individual fibers in the length of broken cable on a second side of the break. This needs to be done correctly so that each individual fiber in the first length of broken cable is re-connected to the corresponding individual fiber in the second length of broken cable. (For each fiber in the first length of broken cable the corresponding fiber in the second length of broken cable is the one which was originally part of the same fiber before the cable was broken). The fibers are typically hard to distinguish from each other, so matching and reconnecting the corresponding fibers is a difficult job.
A method of matching the corresponding fibers exists in which an optical signal (in the form of a modulated laser beam) is transmitted down one of the fibers in the first length of broken cable. The signal is transmitted from the end of the fiber at a regeneration station, which is typically some distance from the break, to the end of the fiber at which the break occurred. A handheld receiving device is then used to find the presence of this signal in one of the fibers amongst the bundle of fibers at the break. The procedure is then repeated on the second length of broken cable by sending a signal down the corresponding fiber in the second length of broken cable and using the receiving device to find the signal in one fiber from the bundle of fibers at the break. The broken ends of the fibers identified by the optical signals are then reconnected. It is possible to locate the corresponding fibers at the regeneration stations because all the fibers are still connected there, but it is difficult without this method to match the corresponding fiber ends at the break. The above procedure is carried out for each fiber in turn. An operator is required to change the transmitter onto each fiber in turn at both regeneration stations and to identify and rejoin the fiber ends at the break. This method is time consuming and is prone to error.
Therefore, at its most general, the present invention proposes that signals of different frequencies are input to respectively different fibers, so that each fiber has a unique waveform (frequency signal) thereon. The inputs fed to one end of the fibers are detected at another end of the fibers (e.g. at a break). Since each fiber has a unique signal, it is possible to determine at the detection end which fiber is which, thus identifying the respective fibers. The signals may be applied to the fibers successively or simultaneously.
For practical purposes, it is desirable that the signals input to the fibers are generated from a common source. This could be achieved by generating a clock signal, which is subdivided by successive integers to produce a series of frequency signals subdivided by successive integers to produce a series of frequency signals.
At the detector, it will normally be necessary to perform a Fourier Transform on each signal, to determine its frequency, usually by taking a series of samples of the signal. However, if the computations of the Fourier Transform are to be efficient, it is desirable that the frequencies being analysed represent an arithmetic progression, so that they are in the sequence 1, 2, 3 . . . multiplied by an appropriate constant. However, this creates conflicting requirements at the transmitter and the detector, because if such a sequence of signals is to be received at the detector, and if those signals are produced by integer subdivision of a master clock, the clock frequency must be very high if a large number of signals are to be produced. Thus, for example, if five signals are to be produced in the sequence 1, 2, 3, 4, 5, the master clock must be divided by 60 to produce the first, 30 to produce the second, 20 to produce the third, 15 to produce the fourth and 12 to produce the fifth, and a very high number would be needed to produce more signals. Since a cable will have many fibers, impractically high clock frequencies may be needed. The alternative to such a high clock frequency if integer subdivision is used, is to compromise the computations of the Fourier Transform at the detector end.
Therefore, it is desirable for the signals applied to the different fibers to be generated in a way which permits them to have an arithmetic progression of frequencies, but to produce them without integer subdivision of a master clock signal. A development of the present invention seeks to achieve this and proposes that clock cycles are counted in groups of X at modulus (or base) Y. Then, each time a count of X results in the modulus Y being reached or exceeded, the state of the output signal is changed. X is an integer which is less than Y and which may then be different for each waveform, so that a set of integers Xn are used to generate the waveforms and can be made to be members of an arithmetic progression by suitable selection of Xn and Y. The waveforms will have a repeat period equal to 2Y.
To understand this invention, consider the simple case where Y is 10 and the values of X for three signals are 4, 6 and 8. If, each time the signal passes 9, (0 to 9 forming the set of Y integers) the value of the output signal is changed, the effect is to generate three signals with frequencies which are in the arithmetic progression 2, 3, 4. That is achieved because the mark ratio of the signals is not 50/50, but instead has a small variation within the signal and from one signal to the other. However, when the resulting signals are sampled and then analysed by Fourier Transform, the effect of that variation is eliminated. Thus, it is possible to generate an arithmetic progression of frequency signals using a smaller clock frequency than would be necessary if signals were to be produced by integer subdivision.
If signals are produced whose frequencies are an arithmetic progression, and those signals are then applied to respective fibers of an optical fiber, a detector using Fourier Transforms can operate efficiently, and therefore quickly. The signals produced may thus have a common repeat time, and the number of cycles of each waveform in that time is equal to the number of cycles which would have been undergone by a square wave signal of the same frequency having a 50/50 mark space ratio. However, as mentioned above, the signals produced by this development of the present invention do not have a 50/50 mark space ratio but their frequencies still correspond to such signals.
In this development of the present invention, the detector will need to sample the signals for a period of time that is the repeat period of the transmitted waveform, or multiples of it.
In practice, although the example of this development of the present invention mentioned above used values of Y=10 and values of X=4, 6 and 8, the numbers used in a practical device will be higher. The reason for this is to permit a suitably large range of frequency signals. For example, if Y is 131072, Xn may be 256, 257, 258 . . . 511.
The signals may be produced by generating light from a laser or LED, and then modulating that light using a modulator or modulators operating to generate light signals having variations based on the principles discussed above. If the light is to be applied successively to the fibers of the cable, then a single modulator may be used with the modulation frequency changing, or multiple modulators may be used modulated at different frequencies for simultaneous arrangements. Whilst it is normally convenient for that modulation arrangement to be permanently installed at a suitable location on the cable, it may be possible to provide a device which can be positioned on the cable when needed.
The present invention relates to a method of identifying fibers of a cable as discussed above, and also to an apparatus for carrying out such a method. In such an apparatus, an accumulator or register of size Y may be used so that the state of the respective signal is changed each time that accumulator or register overflows. Since the register or accumulator operates on digital principles, it is convenient if Y is a power of 2.
There may also be provided a method of generating a set of waveforms for modulating physical signals, said waveforms approximating pure tones, the frequencies of said pure tones being members of an arithmetic progression, the method comprising the following steps:
a) establishing a clock cycle
b) defining a repeat period for the waveforms, the repeat period being equal to 2Y cycles of said clock and being the same for each waveform
c) for each waveform counting X modulus Y at each clock cycle and changing the state of said waveform each time Y is reached or exceeded, X being an integer which is less than Y and which is different for each waveform, the set of integers X used to generate said waveforms being composed of members of an arithmetic progression.
The signals are then applied to an optical fiber cable with each signal applied to a different fiber.
There may also be provided apparatus for generating a plurality of modulated optical signals comprising
a) a clock for establishing a clock cycle
b) light generating means for generating a plurality of optical signals
c) modulation waveform generating means regulated by said clock for generating a plurality of modulation waveforms
d) modulating means for modulating said plurality of optical signals generated by said light generating means with said plurality of modulation waveforms generated by said modulation waveform generating means;
said waveform generating means being configured to generate said plurality of waveforms by
i) defining a repeat period for the waveforms, the repeat period for each waveform being equal to 2Y cycles of said clock
ii) for each waveform counting X modulus Y at each clock cycle and changing the state of said waveform each time Y is reached or exceeded, X being an integer which is less than Y and which is different for each waveform, the set of integers X used to generate said waveforms being composed of members of an arithmetic progression. Again, the signals are applied to respective fibers of an optical fiber cable.
The waveform generating means may have a plurality of adding means, a plurality of accumulators of size Y, a plurality of overflow detecting means and a plurality of waveform output means for outputting a plurality of waveforms. Each adding means may then be linked to a respective accumulator and configured to add a given integer X to said accumulator every clock cycle, a different given integer X being used by each adding means, the set of integers X, used by said plurality of adding means being composed of members of an arithmetic progression. Each respective accumulator is then linked to a respective overflow detecting means, and each respective overflow detecting means is linked to a respective wave output means. Then the respective overflow detecting means can detect when the accumulator to which it is linked overflows and communicate this to the waveform output means to which it is linked. The waveform output means can then change the state of the waveform, which it outputs each time the overflow detecting means to which it is linked communicates that an accumulator has overflowed.